The present invention relates to image production by nuclear magnetic resonance (NMR), which is at present undergoing rapid development, particularly for the examination of the human body, whereof it makes it possible to obtain images or pictures with a quality and precision unknown hitherto with conventional X-ray radiology methods.
The NMR image production method uses the property of nuclear magnetic resonance of certain nuclei present in the human body, essentially the protons distributed throughout the organism and several nuclei which are of biological interest and present in much larger numbers, such as phosphorus (.sup.31 p), potassium (.sup.39 k) and sodium (.sup.23 Na).
A medical NMR installation essentially comprises a magnet for producing throughout the zone of the body to be examined a static, uniform polarization magnetic field Bo, on which is superimposed with the aid of an auxilliary coil a radio-frequency rotary field in a plane perpendicular to the direction of the preceeding field Bo.
The images are obtained by resonating the hydrogen nuclei or protons contained in the biological tissues. This resonance is possible because each proton behaves like a microscopic magnet. Thus, on applying the static field Bo, the spins are all oriented parallel to the field axis. Thus, they only have two possible orientations, in the direction of the field or in the opposite direction. Nothing happens when the rotary field is applied at a random frequency. However, if this frequency is chosen equal to or very close to f.sub.o, such that 2.pi.f.sub.o =.gamma.B.sub.o, in which .gamma. is a physical constant characteristic of the nuclei which it is wished to resonate and called the gyromagnetic ratio, the coupling of the spins and the rotary field reaches a value such that the latter resonate.
The detected resonant signal is proportional to the magnetization of the nuclei placed in the magnetic polarization field Bo. The observation of the resonance phenomenon consequently requires the presence of a high magnetic field (0.1 to 1 Tesla) with a very high uniformity throughout the volume to be imaged.
The actual magnetic resonance image, which constitutes the medical objective of the patient scan, is then progressively constructed by spatial coding with the aid of a computer. For the purpose, several known methods exist, particularly the construction of the image by points, by lines and by planes and there is no need to give details thereof here. For the understanding of the present text, it is merely necessary to know that they all involve the sperimposing in time succession on the polarization field Bo of a large number of complementary fields having very short durations and amplitudes, which are variable in three directions (x, y and z) in space in order to produce with Bo, a resultant field B, whose component Bz has spatial gradients .differential.Bz/.differential.x, .differential.Bz/.differential.y and .differential.Bz/.differential.z, which are constant along x, y and z, z being the direction of the field Bo. The associated electronics and antennae for the reception of the various resonant signals of the protons have a sufficiently wide path band to receive the resonant frequency f, close to f.sub.o, corresponding at each point examined to the value 2.pi.f=.gamma.B, B being the modulus of the resultant field at this point, by definition close to Bo.
For information purposes, field Bo has a value of 0.1 to 1 Tesla and the gradients .differential.Bz/.differential.x, .differential.Bz/.differential.y and .differential.Bz/.differential.z have an order of magnitude of 10.sup.-1 to 10.sup.-2 Tesla/meter over an average length.+-.approximately 20 cm. Also for information purposes, roughly 6 minutes are required by this method to construct a section of 128.times.128 points, which corresponds to the establishment of 128 different coding gradients. Modern machines make it possible to obtain several sections simultaneously (4 to 6).
The magnet used for producing the polarization magnetic field Bo can be, as a function of the particular case, either an electromagnet with or without a magnetic circuit, or a permanent magnet, each of the solutions having specific advantages and disadvantages.
In the case of an electromagnet, the latter usually comprises simple helical coils in air, with no magnetic circuit, whereof the diameter of the winding is made sufficient to surround the cranium of the patient or his body and all the necessary accessories (antennae, gradient coils, etc). These coils then produce a polarization field Bo, which passes through the patient from his feet to his head in the axis of his body.
In the case of a permanent magnet, the presence of a magnetic coil makes it necessary to place the patient in a tunnel defined by the magnet casing and the polarization induction Bo then generally passes through the patient's body from the chest to the back in a sagittal direction.
The present invention specifically relates to the field of devices for producing linear magnetic field gradients in permanent magnets or electromagnets, having a high permeability pole pieces and more particularly applies, although this is not limitative, to permanent magnets of the type forming the subject matter of the French Patent Application No. 2562785 published on Oct. 18, 1985 and whose structure will be referred to hereinafter. Thus, the invention applies to the case where the patient placed in the air gap between an upper pole piece and a lower pole piece is traversed by the polarization field in a sagittal direction identified in axis Oz.
The devices for producing magnetic fields with constant gradients constitute a very important part of an image production device for display and condition its structure because:
(1) the quality of the gradients is dependent on the quality of the image, any variation in the gradient leading to the distortion thereof; PA0 (2) the devices, generally coils, must be positioned as close as possible to the object to be imaged in accordance with imperative linearity conditions (to within 1% throughout the colume to be imaged), in order to limit the magnet air gap and consequently the weight and therefore the cost of the machine, this being in spatial configurations such that they permit the passage of the patient in said air gap; PA0 (3) the method for producing the constant gradient fields must be integrated in an optimum manner in the structure of the magnetic circuit (in the present case in the pole pieces) to limit the power of the pulsating current supplies, which can reach several dozen kW in existing displays (resistive and cryomagnets); PA0 (4) the constant gradient fields must be established for each measurement in relatively short times, less than a few ms.
For NMR image production systems with resistive or superconductor magnets constituted by coils, whereof the field lines re-close in air, for producing complementary fields producing constant spatial gradients, correction coils are used, designed for correcting in a general manner the non-uniformities of the polarization field up to the fourth order, order by order, with a set of coils for each order and the gradients are likened to corrections of the first order.
All these systems are based on the fact that any physically realisable magnetic field can develop in accordance with a basis of orthogonal functions, usually Legendre spherical harmonics. This process makes it possible to profile the field to different orders, the gradient being an approximation of the field of the first order. The advantage of this method is that, in theory, the different parameters of settings associated with a certain order (e.g. an electric current in a set of coils) do not interact and the checking of the field can be carried out in a progressive, although tedious manner, up to very high orders.
These devices are more particularly described in the article "Magnet field profiling: Analysis and correcting coil design" published in "Magnetic Resonance in Medicine", 1, 44-65, 1984. They are constituted by coils mounted on cylinders surrounding the patient and, on wishing to apply this procedure to permanent magnets, it will be necessary to place the coils in the air gap, relatively remotely from the pole pieces, which would lead to an unacceptable reduction in the amount of space reserved in said air gap for the patient's body.
For NMR image production systems with permanent magnets, at present the means used by the designers for producing the linear field gradients are not known, because the designers have maintained secrecy regarding the structures used.
A description will now be given of a known permanent magnet structure to which the present invention can be particularly advantageously applied and which forms the object of the unpublished French Patent Application EN No. 84 05903 of Apr. 13 1984 in the name of the present applicant. This known structure is shown in FIG. 1.
In FIG. 1, which is a perspective front view, there is a permanent magnet system having an upper pole piece 1 and a lower pole piece 2 with a generally rectangular parallelepipedic shape made from a ferromagnetic material with a very high permeability. The structure is laterally completed by two magnetizing blocks 3, 4, which also have a rectangular parallelepipedic shape and which are made from magnetized materials forming the seat of the magnetomotive force for producing the field in the tunnel-shaped air gap 5, specifically defined by the pole pieces 1, 2 and the magnetized blocks 3 and 4. The latter blocks are generally constituted by the juxtapositioning of columns, each constituted by a stack of elementary magnetic bricks such as 3a, 3b, 3c, etc, 4a, 4b, 4c, etc, which are thus also used as spacers and for maintaining the pole pieces 1 and 2 in place. These columns are not necessarily contiguous. The tunnel-shaped air gap 5 intended for optionally receiving all or part of a human body 6, is provided for this purpose with a minimum width and height of approximately 500 mm.
In FIG. 1, arrows show the distribution of the magnetic flux and the field lines in the magnetized blocks 3, 4, as well as in the pole pieces 1 and 2. As indicated in FIG. 1, the magnetic polarization induction Bo in tunnel 5 is directed vertically from top to bottom, i.e. from pole piece 1 to pole piece 2, thus traversing the patient's body 6 in a sagittal direction, 1dentified throughout the remainder of the present text by the setting axis Oz of the reference trihedron Ox, y, z of FIG. 1.
In this configuration, the gradients of component Bz along Oz of the different resulting fields necessary for contructing the image by spatial coding, namely .differential.Bz/.differential.x, .differential.Bz/.differential.y and .differential.Bz/.differential.y must be constant and satisfy the three following equations: ##EQU1## in which Bz is the component of the resultant induction along the previously defined axis Oz and Gx, Gy, Gz are three constant gradients of the resultant induction in the three directions Ox, Oy and Oz.